January transfer window: if your club could sign just one player, who would you choose?

The precious January transfer window is upon us. It’s a time when struggling clubs aim to strengthen their ailing squads, and the in-form big clubs smugly claim to be happy with what they have. What would you do with your chairman’s cash? (No, you can’t afford Ronaldo, Kaka and Messi.)
You can buy just one player, from any club in the world. Who would you sign, and why?